HOW TOMATO KETCHUP (ALMOST) HELPED RAISE THE Q

During a visit to the Mc Michael Rally in the summer (12th July), my attention was caught by an old and very encrusted tuning capacitor. My cursory interest immediately triggered the words “that’s a pound, mate”, an offer I felt I couldn’t refuse. Then, after the deal was done, “I dunno what you’re going to do with it”. I replied with some quip about keeping a door open, but my impulse buy was inspired by a vision of restoring this fine piece of tank engineering to its former glory and putting it to good use in (yet another) magnetic loop antenna. The weight of the thing suggested it was made from something other than aluminium and since it showed no interest in a magnet, I deduced the construction was brass with some kind of alloy cast end plates.



First attempts.
Carrying this beast around the rally for the rest of the day made my arm ache but became a conversation piece with several sellers. One suggested that the vanes might even be silver-plated. Another suggested that a solution of baking powder might do the trick. However, when I arrived home later that evening, I decided to try Cillit Bang as a first line of attack. The dirt began to lift after about 20 minutes and the letters CYLDON CES.TR.25 became visible on the back plate. Then I tried the hot baking soda bath idea. Not a good move. This left the capacitor coated in a white powdery residue that seemed to seal in the remaining dirt and crud. My gut feel was that this needed something acidic.

Blood Everywhere
At the back of my mind was an idea to try a weak solution of hydrochloric acid (sold as ‘Brick and Patio Cleaner’), but how weak should the solution be? Previous experience showed that a 25% solution of HCl will etch a copper PCB in a matter of seconds. Something less potent was required. Then I remembered that vinegar is good for cleaning lime scale on taps etc. and I reasoned that a clinging vinegar based goo would be even better. There was a very old bottle of tomato ketchup the larder and I eagerly set to work shaking the stuff all over the capacitor, working it well between the vanes with a paint brush. The result resembled a bloody massacre and was very satisfying. I left the great red glob in a plastic bucket with a plastic lid to ferment overnight.

Stage 3 – Cleaning off the bloody mess
I looked into the bucket the following morning with less enthusiasm than the night before and realised that some serious cleaning was going to be required to clean off the abomination of blood clots. So I boiled up a water/vinegar solution in an electric kettle. This is a well-known method for de-scaling kettles and turned out to be a good guise to conceal my real motives from the domestic director. It also gained me a few house work brownie points. The downside was that boiling vinegar made the whole house smell like a chip shop and it was fortunate that I did this while the governor was distracted with other duties. I used 1 litre of Sainsbury’s Basics Vinegar (13p per 500mil bottle!) to 500mils of water and left the capacitor to soak in the hot solution for an hour.

It turns out that tomato ketchup is sticky stuff and great globs of it lodged stubbornly between the vanes in the most inaccessible places in accordance with the laws of Murphy. It took a cloth wrapped around a knife to dislodge the more persistent clumps. I began to feel that perhaps the tomato ketchup idea was not such a good idea after all.

Defeated to Disassembly
Having failed dismally in my attempts to short cut the cleaning process, I finally resorted to disassembling the capacitor down to the last nut and bolt and cleaning each part individually with brass/silver polish. This proved to be laborious task but revealed that the capacitor vanes were indeed silver-plated and that the end plates were probably cast from bronze. The final result of my labours is shown in Figure 2.



Was it a worthwhile exercise? Yes, I think so. The result is a fine, battle worthy split stator variable capacitor that should withstand well over 10kV when tuning a magnetic loop antenna. However, next time I’ll definitely pass on the tomato ketchup.

Peter G4FYY